Studies will investigate several mechanisms that may be playing a role in hypertension resulting from the chronic administration of a synthetic estrogen (mestranol) commonly used as an oral contraceptive agent; these studies will include volumes (plasma volume, extracellular fluid volume, total body water), renal functions (insulin and PAH clearances), pressor responses to norepinephrine, and contractile responses of aortic strips to norepinephrine. Experiments also will examine the effect of 12 hours continuous infusion of an angiotensin II antagonist on mean arterial pressure in rabbits with chronic renal hypertension. Additional studies will determine volumes (plasma volume, extracellular fluid volume, and total body water) in normal rabbits, prehypertensive rabbits (3 days after renal artery stenosis), and chronic hypertensive rabbits (30 days after renal artery stenosis); these will be 2-kidney rabbits, i.e. renal artery stenosis of one kidney with the contralateral kidney intact. Studies will be continued on the plasma concentration of a non-renin-angiotensin pressor substance in the plasma of renal hypertensive and normal rabbits.